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Aviation History
1912
1912 - 0873.PDF
SEPTEMBER 28, 1912. 1/jKSHT NEGATIVE PRESSURE ON WINGS. As is evident from a letter that we publish this week relating to Eiffel's Pressure Distribution Curves, there is nothing like a good diagram to bring home to the mind a principle that leaves a reader almost entirely unaffected when expressed in words. Almost from the day FLIGHT was founded we have pursued an elementary theory of the aerodynamics of the aeroplane based on the simple hypothesis that *' the machine is supported in flight by the inertia of the air." The very first deduction from this starting point leads to the necessity of defining the mass of air that the wing in its flight causes to be in simultaneous downward acceleration. The evidence of experimenters from the Wright Brothers onward has afforded convincing proof that part of the said mass is unquestionably located above the wing. In other words, the wing of an aeroplane is not supported merely by the air that is positively deflected by its under side. Its motion through the air deflects downwards a stratum of the atmosphere, the effective depth of which is partly above and partly beneath the wing. This much has been known for a long time and anyone who has stopped to think about it must surely have asked himself the question, how does the top side of the stratum communicate lifting force to the wing ? Obviously, it can only do so by virtue of its existence in a state of negative piessure, that is to say, the region above the wing exerts a " suction." This latter term is apt, however, to convey, a medley of curious ideas to many minds ; it is not easy to give quite the impression that one might wish with regard to the nature of this negative pressure region. One simile that is sometimes helpful is to imagine the upper side of the wing to be covered with a sheet of material that is stuck to its surface. In order to remove the material it will be necessary to pull upon it, which will place all its strands in tension and also exert a tension upon the wing itself. If the material is of several layers in thickness the stress will be com municated from layer to layer. In some such way as this it is, perhaps, possible to get a materialised picture of the air above the wing and to crystallise in the mind an idea of negative pressure and its manner of exerting a lift. The important point in this connection is that there is no need to assume the assistance of vortices, or any other peculiar phenomena, any more than there is to assume that our imaginary piece of material must have rucks in it. The purpose of a wing, according to the hypothesis, is to deflect a stratum of air, and the less disturb ance it causes when doing so the higher is the potential efficiency of its action. Such experimental evidence as exists shows that the lower surface of the wing causes the air stream to follow its own contour, but that the upper surface fails in this respect. The general character of the flow over the upper surface is, in fact, one in which the air stream breaks away from ithe contour of the wing surface and forms rollers or vortices above it. When the attitude of the wing assumes a large angle of incidence, then the region behind the wing in the vicinity of the upper surface becomes " dead water." In any case, the region above the wing is a region of negative pressure, and would be a region of negative pressure just the same if the upper part of the stratum evenly followed the contour of the upper surface of the wing. Wherever there is tension in the fluid, and tension is obviously required in order to lift the wing from above, the stress in the air must cause negative pressure. But—and this seems to be a point on which the minds of so many students diverge from the logical sequence of the argument that is based on our fundamental hypothesis—the existence of a negative pressure or otherwise does not affect the main fact that the total lift results solely from the acceleration of the mass of air deflected. The simplest case for which to work out the problem is, obviously, one in which the wing is assumed to deflect a stratum of fluid of strictly uniform density and viscosity and having a well-defined boundary. Such a state of things does not, as everyone knows, actually exist, but whatever the modifications may be, however vague may be the "sweep" or depth of the stratum, for example, there must always be some equivalent depth of our hypothetical stratum that would produce the same result. In the same way, it is possible to analyse the question of deflection ; the air does not actually follow the precise contour of the wing, but still there must be some equivalent angle of deflection that would produce the same result were our hypothetical stream to flow uniformly through it under the assumed conditions. These equivalent conditions are true and justifiable so long as the hypothesis that an aeroplane is supported in flight by the inertia of the air holds good. By keeping that hypothesis steadily in view the whole problem must in the end be confined solely to an expression of mass and acceleration. Intermediate stages concerned with the nature of the flow and the pressure distributions are digressions for the purpose of increasing the particular knowledge relating to certain aspects of the subject, in order that the assumptions necessarily made at many stages of the main argument may be less wide of the mark. Thus, the fact that KiffePs pressure distribution curves show a negative pressure, should occasion no surprise ; but as an indication of the detail character of the pressure distribution they are invaluable, always assuming, of course, that they are accurate. Accurate ot not in the strict sense of the term, however, there is no doubt whatever that Eiffel's pressure distribution curves are of the greatest possible value in helping students to fix their ideas, in giving them n basis on which to work out specific examples and in indicating in an unmis takable way certain general principles that, being common to all the experiments, are likely to remain true, even should subsequent research tend to modify some of the details of Eiffel's work. It was to these broad principles that Mr. Handley Page mainly confined his attention on the occasion of the Aeronautical Society's meeting and for which work many students must remain indebted to his aid, for Eiffel's book "La Resistance de l'Air" is, as yet, only available in the original Erench, and we are unaware of any effort that is being made to translate it, although we are of the opinion that at the present time there does not exist a book bettei worth translating if only it is done quickly. ® ® ® ® I w l. The beautiful memorial window placed In the church at Eastchurch to commemorate the late Hon. Charles S- Rolls and the late Mr. Cecil Grace. 873
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